1. Field of the Invention
The present invention relates to a support sheet for photographic printing paper. More particularly, the present invention relates to a support sheet for photographic printing paper, in which the peel strength between a substrate sheet and a polyolefin resin coating layer formed on a surface of the substrate sheet is controlled to an appropriate level for peeling or stripping.
2. Description of the Related Arts
A conventional photographic printing paper (hereinafter referred to as "photographic paper") has a multiple layer structure comprising a substrate sheet, a polyethylene resin layer, containing a light-reflecting substance, coated on one surface of the substrate sheet, a photographic emulsion layer formed on the polyethylene resin layer, and another coating layer, comprising a polyethylene resin, formed on the opposite surface of the substrate sheet. A photographic paper having an undercoat layer formed between the polyethylene resin layer and the photographic emulsion layer for improving the adhesion between the two layers is known. The photographic paper as mentioned above is printed and developed, and colored or black-and-white images are formed on the photographic emulsion layer of the photographic paper. The images recorded on the photographic emulsion layer can be clearly observed due to the light reflection off the light-reflecting substance, for example, titanium dioxide (TiO.sub.2) particles, contained in the polyethylene resin layer, and on the substrate sheet surface.
In various uses, the printed and developed photographic paper sheets are adhered to a notebook, a book, a desk, a show-window, a post card or other article, and utilized for display and advertisement. Accordingly, a development of a new type of photographic paper suitable for the above-mentioned use is desired.
In conventional photographic paper sheets as mentioned above, a thick paper sheet is used as a substrate sheet to prevent a breaking or bending thereof, and therefore, when the printed and developed photographic paper sheet is adhered to a notebook, a book, a desk, a show-window, a post card or other article as a sticker or seal, the thickness of the resultant photographic print-bonded article is greatly increased, or it becomes difficult to firmly adhere the entire surface of the photographic print to the article because the support sheet has curled. As a general attempt to solve this problem, a reduction of the thickness of the substrate sheet in the support sheet for the photographic paper has been considered. In this case, however, the stiffness of the photographic paper is lessened and the curl balance thereof is lost, and thus it cannot be used in the conventional photographic developing apparatuses.
Even if an attempt is made to use a special thin paper sheet as a support sheet for the photographic paper, this attempt will be disadvantageous from the practical and economical viewpoints.
The conventional photographic paper sheet consists of a substrate sheet, polyolefin resin coating layers formed on the surfaces of the substrate sheet, and a photographic emulsion layer formed an one of the coating layers. Therefore attempts have been made to provide a photographic paper sheet in which the photographic emulsion layer can be peeled from the substrate sheet together with the polyolefin resin coating layer located thereunder, by utilizing the above-mentioned structure to eliminate the disadvantages of the conventional photographic paper.
That is, Japanese Unexamined Patent Publication No. 54-92219 discloses that, in a process for the preparation of a support sheet for a photographic printing paper, lamination conditions at the step of coating the substrate sheet with a polyolefin resin containing a light-reflecting substance are appropriately selected and controlled, so that the adhere strength between the substrate sheet and the polyolefin resin coating layer is appropriately controlled, and the photographic emulsion layer in the photographic paper can be easily peeled, together with the polyolefin resin coating layer, from the substrate sheet.
In this attempt, however, slight variations in the conditions when preparing the support sheet for the photographic paper, result in variations in peel strength between the substrate sheet and the polyolefin resin coating layer, and it is difficult to maintain this peel strength at a constant value. Therefore, it is practically difficult to obtain a photographic paper in which the photographic emulsion layer can be peeled together with the polyolefin resin coating layer from the substrate sheet, if necessary.